1. Field of the Invention
This invention resides in the field of electrically conductive ceramics, and incorporates technologies relating to nanocrystalline materials, carbon nanotubes, and sintering methods for densification and property enhancement of materials.
2. Description of the Prior Art
The ability of ceramics to withstand extreme conditions of temperature, mechanical stress, and chemical exposure without failure or with a very low failure rate has led to the use of ceramics in applications that require high-performance materials, such as heat engines, cutting tools, wear and friction surfaces, and space vehicles. In recent years, the use of ceramics has extended into the fields of microtechnology and nanotechnology, since the increasing demands of nano-scale electronics and microelectromechanical systems (MEMS) for example have prompted researchers to investigate the use of ceramics in these areas as well.
An unfortunate characteristic of nanocrystalline ceramics is brittleness. To reduce the brittleness, composites have been developed in which secondary materials are dispersed throughout the ceramic matrix. In some of the more recent developments, carbon nanotubes, specifically multi-wall carbon nanotubes, have been used as the secondary material. A description of “ceramic matrix nanocomposites containing carbon nanotubes” is found in Chang, S., et al. (Rensselaer Polytechnic Institute), U.S. Pat. No. 6,420,293 B1, issued Jul. 16, 2002 on an application filed on Aug. 25, 2000. While the description encompasses both single-wall and multi-wall carbon nanotubes, the only carbon nanotubes for which test data is presented in the patent are multi-wall carbon nanotubes. To form the composites, the starting powders in the patent are sintered into a dense continuous mass by hot isostatic pressing. Single-wall carbon nanotubes, although not investigated to the extent of multi-wall carbon nanotubes for this purpose, are known to have both high stiffness (a Young's modulus of 1,400 GPa) and high strength (a tensile strength well above 100 GPa).
In addition to applications where their mechanical properties are needed, ceramics are of increasing interest in electronics since various kinds of electrical devices are being designed for use in environments that require a combination of high temperature resistance, toughness, and chemical inertness. In the microelectronics industry, for example, materials with the qualities demonstrated by ceramics are sought for use as silicon substitutes, as trays and wafer carriers, as ruggedized microchip substrates, and as components with electrostatic discharge protection. In the microwave industry, the high-temperature environments that are frequently encountered require high-performance materials that can shield components from, or absorb, electromagnetic interference. In the automotive industry, high-temperature, high-strength, chemically inert materials that conduct electricity are needed for components such as fuel injector assemblies. The need for these qualities extends to medicine as well, where a wide variety of medical devices, such as implants, prostheses, and surgical devices, would benefit from a combination of electrical functionality, high strength and chemical inertness. In electrical power supplies such as batteries and solid oxide fuel cells, electrodes that possess these properties are needed. The need also exists in analytical and testing devices for materials used as chemical sensors, gas separation materials, and materials for hydrogen absorption. And in the aerospace and defense industries, materials with these properties are needed for aircraft and aircraft engines as well as for thermal management materials in human spaceflight applications.
Ceramics are electrically insulating materials. To make them electrically conductive, ceramics have been formulated as composites with electrically conductive fillers. Carbon nanotubes have been investigated as conductive fillers since carbon nanotubes are known to possess both high electrical conductivity and high thermal conductivity. Studies of the electrical characteristics of ceramic composites that contain carbon nanotubes, notably composites of alumina, iron and carbon nanotubes, composites of magnesium oxide, cobalt and carbon nanotubes, and composites of MgAl2O4, iron, cobalt, and carbon nanotubes have been reported by Flahaut, E., et al., “Carbon Nanotubes-Metal-Oxide Nanocomposites: Microstructure, Electrical Conductivity, and Mechanical Properties,” Acta Mater. 48: 3803-3812 (2000); Laurent, Ch., et al., “Carbon Nanotubes-Fe-Alumina Nanocomposites. Part II: Microstructure and Mechanical Properties of the Hot-Pressed Composites,” J. Euro. Ceram. Soc. 18: 2005-2013 (1998); Peigney, A., et al., “Carbon Nanotubes-Fe-Alumina Nanocomposites. Part I: Influence of the Fe Content on the Synthesis of Powders,” J. Euro. Ceram. Soc. 18: 1995-2004 (1998); Peigney, A., et al., “Carbon Nanotubes in Novel Ceramic Matrix Nanocomposites,” Ceram. Inter. 26: 677-683 (2000); Peigney, A., et al., “Carbon Nanotubes Grown in-situ by a Novel Catalytic Method,” J. Mater. Res. 12: 613-615 (1997); Peigney, A., et al., “Aligned carbon nanotubes in ceramic-matrix nanocomposites prepared by high-temperature extrusion,” Chem. Phys. Lett. 352: 20-25 (2002). The nanocomposites in these reports were produced by hot pressing nano-sized powders. The composites were electrically conductive to a moderate degree with an electrical conductivity within the range of 0.2-4.0 S/cm. The fracture strengths and fracture toughnesses of the composites were generally lower however than those of the metal-ceramic composites (lacking carbon) and only marginally higher than those of the pure ceramics. The Peigney et al. 2002 paper (Chem. Phys. Lett. 352: 20-25 (2002)) also reports that the carbon nanotubes in the composites can be aligned in the bulk ceramic matrix of the composite by a high-temperature extrusion technique to produce materials that show an anisotropy of electrical conductivity. The best conductivity however was only obtainable in the center of the extrusion since the carbon nanotubes in other parts of the composite had been damaged during the extrusion.
Of further relevance to this invention is the literature on electric field-assisted sintering, which is also known as spark plasma sintering, plasma-activated sintering, and field-assisted sintering technique. Electric field-assisted sintering is disclosed in the literature for use on metals and ceramics, for consolidating polymers, for joining metals, for crystal growth, and for promoting chemical reactions. The densification of alumina powder by electric field-assisted sintering is disclosed by Wang, S. W., et al., J. Mater. Res. 15(4) (April 2000): 982-987.